The LISA observatory project will use three test masses, each with its own shield, to detect gravitationnal waves.

The purpose of the LISA and of a Pioneer effect probe is different: the LISA will have to detect much weaker effects, using laser interferometry, while a Pioneer effect probe can be accurate enough with radio location. The LISA would not be bothered with a permament offset in the results, as it is meant to detect only transcient or periodic effects. But such a permanent offset would mess up a Pionner effect probe, as it studies signals which are constant over years.

For this reason I proposed my rotative shield design to cancel any permanent offset. This cannot be used by LISA. So they are developping a very accurate tracking system: 10 nanometres! Figure that this is better than welding the shield to the test mass with steel rods. Eventually this technology may be better than my rotating design to cancel even permanent offsets. So re-using the LISA technology may save most of the development costs for a Pioneer effect probe.

All of these approaches use assumptions we should not be making. If there is a variation in the permeabiliy of space to mass necessary to explain the phenomena I outlined above, there is almost certainly a corresponding gradient in the speed of light.

This can be demonstrated with the Galileo paradox:

Put Galileo on the Dark side of Mercury, rolling his balls to measure the gravitational constant. Because of the nearby mass of the sun, the balls will roll slower, which would cause Galileo, not knowing about GR, to underestimate the G constant. EXCEPT Galileo's clock is also ticking slower, so the value would be very close to correct.

Watching the experiment from an Earth frame of reference, the balls would appear to roll slower, and a GR space curvature correction must be used to explain the phenomena.

Three things: 1) The same observations can be interpreted as a time or space dilation, depending upon the frame of reference. 2) Performing the experiment on Mercury, Galileo could be completely oblivious to the need for GR to explain the results from Earth. 3) It is relatively simple to transfer both frames of reference to a single coordinate system where the absolute pathlength through a given volume of space varies as a function of mass.

Magueijo eluded to this transformation in Faster Than the Speed of Light, where he found it difficult to prove his theory required a new physical concept; and not just a transformation of GR into a completely compatible coordinate system, where pathlengths and the speed of light vary, not time and space curvature. (I am of the opinion that this mathematic transformation provides a better conceptual bases for GR phenomenon.)

So any attempts to measure unknown or poorly characterized forces must also address an untested assumption in measurement theory: the Speed of light is an absolute constant that is not mass dependent; or more exactly: Current GR perameters correctly compensate for mass-dependant effects upon light.

Again, existing solar probes have tightly constrained any deviations from established GR constraints: Bertotti has used Cassini to constrain unexpected GR variance to a factor of 2.3x10^-5 near the Earth's orbit. Perhaps the best solar constraint on the speed of light is the Pioneer Anomally itself - 8x10^-9m/s^2, but this is only beyond the obital distance of Saturn. (Notice that since we use the two-way speed of light to determine the position of the Pioneer probes, the acceleration of the probes could be away from, rather than towards the sun, as long as it is of the same magnitude as any change in the speed of light.)

Again, much of the science needed to nail down these possible discrepancies can be extracted from the current generation of probes, but only if the experimentors are aware of the unbridled parameters and the need for additional onstraints.

Edited to add:

One more question about LISA - unless and until the current LIGO generation of gravity antenna detect ANY gravitational phenomena, should we be vesting in another experiment? IAOTO the waves do exist, but we may be searching with the wrong kind of antenna.

One more question about LISA - unless and until the current LIGO generation of gravity antenna detect ANY gravitational phenomena, should we be vesting in another experiment? IAOTO the waves do exist, but we may be searching with the wrong kind of antenna.

Well LISA will be serching in a completely different frequency band. A band which should include waves from binary neutron stars which pretty much must exists given current observations (and at a known amplitude), unlike LIGO which can only detect much more exotic and theoretical objects and mergers. So yes I do think it's worth investing in, even given the non-detections at LIGO.

Heading off topic but...Well LISA will be serching in a completely different frequency band. ..

Damn! I'll say we need LISA, yesterday, not too many years from now. Any chance of bumping LISA ahead of PLANCK? The CMB has a local contamination issue that needs to be resolved to reathenticate, if possible, the accuracy of the WMAP results.

But A drag-free triangulated laser ranged probe orbiting the Sun will also provide constraints upon Pioneer-like acceleration anomalies if they effect lasar ranging.

What would be fine is if somebody have the log-log plot, amplitude versus frequency, with expected domain for each gravitationnal wave source, and the expected sensitivity of each instrument.

What I heard (to check) is that the LIGO gravitational wave observatory is curently reaching its full sensitivity, but it still detected nothing (the only thing it could detect, neutron stars spiraling, would happen only once a year in average).

What would be fine is if somebody have the log-log plot, amplitude versus frequency, with expected domain for each gravitationnal wave source, and the expected sensitivity of each instrument.

Yes, that was what I was looking for yesterday, but couldn't while rushing round. Couldn't have been looking very hard as a quick search this morning and, ta-dar!

I think that curve is for Advanced LIGO , Standard LIGO is about one order of magintude less sensitive.

QUOTE (Richard Trigaux @ Sep 6 2005, 04:37 PM)

What I heard (to check) is that the LIGO gravitational wave observatory is curently reaching its full sensitivity, but it still detected nothing (the only thing it could detect, neutron stars spiraling, would happen only once a year in average).

Yes, I don't think we need to start rethinking gravitational wave theory just yet, it's not too surprising that nothing has been detected by LIGO so far. Lets wait for Advanced LIGO first (2009)

Yes, that was what I was looking for yesterday, but couldn't while rushing round. Couldn't have been looking very hard as a quick search this morning and, ta-dar!James

Thanks very much James it is exactly what I wished!!! This graphics tells us exactly what we can expect or not!!

QUOTE (jamescanvin @ Sep 7 2005, 12:27 AM)

I think that curve is for Advanced LIGO , Standard LIGO is about one order of magintude less sensitive.Yes, I don't think we need to start rethinking gravitational wave theory just yet, it's not too surprising that nothing has been detected by LIGO so far. Lets wait for Advanced LIGO first (2009)

James

Yes, no need yet to rethink the gravitationnal wave theory, as LIGO today is only able to detect rare events, mainly neutron stars and black hole coalescence, and only the stronger.

Black holes coalescence is, I think, something very well understood (in the context of General realtivity. But even without relativity we can expect that star-sized masses spiraling a high speed will produce strong gravitationnal effects.). Neutron star coalescence and super nova core collapse are slightly less understood (especially SN core coalescence may be highly disordered and unsymmetrical) but the theory is still reliable. So it is expectable that we detect some events before 2009, and only some years after this date, if we detect nothing, the gravitationnal wave theory is at risk.

Once again, I'm asking a question that I probably ought to just Google up for myself, but it does go along with the thread...

One of the experiments in the Apollo 17 ALSEP was the Lunar Surface Gravimeter. As I recall, it was designed to detect gravity waves. (It failed because it was balanced in 1G and was entirely out of balance, and hence useless, in 1/6G.)

Does anyone know what types of waves the LSG was designed to detect? Would it have been more in the LISA range or the LIGO range?

I guess I'm wondering what kinds of things we might have been gathering data on for more than 30 years if the instrument had just been designed properly...

-the other Doug

--------------------

“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain

I'd have to check, but I think the Apollo 17 instrument's name included the term "Tidal". They were looking at freequencies below those the seismometer would detect, at least in part... looking for whole-moon "ringing" frequencies, like the ringing of the whole earth after a Richter 8+ quake.

The instrument failed because it was balanced in 1 G with the aid of a loading mass or spring which was unloaded on the moon. The problem was an arithmetic booboo in the calculation of the design for lunar gravity.. the instrument had a "bias" range that was adjustable for a range of lunar gravities, or really more accurately, for a range of instrument sensitivities... The adjustable range of the instrument was such that actual lunar gravity (very well known) was outside the adjustment range. This is similar to the focus failure on Deep Impact's hi rez camera.. The camera was "focussed" from pre-launch out of focus conditions by heating the carbon-composite truss to drive out water vapor in vaccuum, intending to slow down and stop when the instrument approached and achieved perfect focus..... it never got there due to a ground base calibration problem. The actual focus point was outside of the adjustment range.

The instrument failed because it was balanced in 1 G with the aid of a loading mass or spring which was unloaded on the moon. The problem was an arithmetic booboo in the calculation of the design for lunar gravity.. the instrument had a "bias" range that was adjustable for a range of lunar gravities, or really more accurately, for a range of instrument sensitivities... The adjustable range of the instrument was such that actual lunar gravity (very well known) was outside the adjustment range. This is similar to the focus failure on Deep Impact's hi rez camera.. The camera was "focussed" from pre-launch out of focus conditions by heating the carbon-composite truss to drive out water vapor in vaccuum, intending to slow down and stop when the instrument approached and achieved perfect focus..... it never got there due to a ground base calibration problem. The actual focus point was outside of the adjustment range.

Oooh, dear! Don't let a certain scientist-astronaut know, this may have wasted some of that precious EVA time during which rocks could have been examined. I can just imagine a series of unexplained murders, with the victims attended to with a balance spring tied between a gnomon and a lunar rake...

Does anyone know what types of waves the LSG was designed to detect? Would it have been more in the LISA range or the LIGO range?

I guess I'm wondering what kinds of things we might have been gathering data on for more than 30 years if the instrument had just been designed properly...

-the other Doug

I think that, even if such an instrument was properly designed, it had far from enough sensitivity to detect expected gravitationnal waves. Gravimeters are very sensitive indeed, they can detect such "low" masses as mountains, and even less (remember the historical Cavendish experiment which measured the effect of a some kilograms mass). But this is very far from enough to detect gravitationnal waves, which are many orders of magnitude under this level of sensitivity. Otherwise it would not be necessary to build such complicated experiments as LIGO, it would be enough to send a gravimeter in the ISS.

Perhaps the most powerfull recent gravitationnal event was the supernova in 1978, but who knows what happens in the gravitationnal field.

When the gravimeter was proposed, selected and designed, the PI (Weber, I think) was claiming or about to claim possible detections of grativational waves with ultrasensative suspended cylinders. The consensus then and now was that plausible sources of waves in the frequency range that the sensors could detect were many orders of magnitude too small to detect (at least at distances out where there was any chance of an event occurring). But theory in 1970 was much less precise than now, too. By observing natural tidal oscillations of the moon and very low frequency seismic signals, they hoped to have a valuable experimet regardless of whether the "blue sky" gravity wave search was a bust or not.

When the gravimeter was proposed, selected and designed, the PI (Weber, I think) was claiming or about to claim possible detections of grativational waves with ultrasensative suspended cylinders. The consensus then and now was that plausible sources of waves in the frequency range that the sensors could detect were many orders of magnitude too small to detect (at least at distances out where there was any chance of an event occurring). But theory in 1970 was much less precise than now, too. By observing natural tidal oscillations of the moon and very low frequency seismic signals, they hoped to have a valuable experimet regardless of whether the "blue sky" gravity wave search was a bust or not.

Yes I remember that there was hopes to find gravitationnal waves with large aluminium cylinders which may resonate and amplify the wave signal. The first experiments to find gravitationnal waves started with such detectors, but they never produced anything. But the search was open...

Oooh, dear! Don't let a certain scientist-astronaut know, this may have wasted some of that precious EVA time during which rocks could have been examined. I can just imagine a series of unexplained murders, with the victims attended to with a balance spring tied between a gnomon and a lunar rake...

Oh, that particular scientist-astronaut was well aware of the problem -- for one thing, when the PI found his instrument wouldn't uncage, he *insisted* that this particular scientist-astronaut must have deployed it improperly, must not have leveled it right. So Houston told him to go back and re-level the experiment -- three times. When told it would not uncage, Schmitt even kicked it, hard, and then re-leveled it again. It still did not uncage.

I'd have to check, but I think the Apollo 17 instrument's name included the term "Tidal".

Not in the experiment title, no -- Apollo 17 carried two gravimeters, the Lunar Surface Gravimeter (LSG) and the Lunar Portable Gravimeter (LPG). The tidal reference may have been in the detailed description of the LSG, but it was not part of the instrument's name.

The LPG was the same type of instrument used by oil companies to find salt domes underneath otherwise flat land -- oil and gas are often entrained in salt domes. It detected negative anomalies on the slopes of the massifs and positive anomalies on the valley floor, indicating just how much more massive the basaltic valley fill is when compared to the massifs. IIRC, the anomalies were on the order of 10 to 30 milligals.

So, the LSG was an ultra-sensitive seismometer that hoped to use the entire mass of the Moon to detect gravity waves? Interesting... even if we now think that gravity waves would have been undetectable with such an instrument.

-the other Doug

--------------------

“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain

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